1. Field of the Invention
The present invention relates to a color image forming apparatus of a system having a plurality of photo-conductive drums and more particularly to a color image forming apparatus for starting reading of the next document before a recording medium passes through all the photo-conductive drums, thereby improving the processing capacity per unit time.
2. Description of the Related Art
Recently, digital color copying machines have been widely developed and spread rapidly. These digital color copying machines have a photo-conductive drum unit for forming an image for each ink color component such as a magenta cyan, and yellow.
Such a digital color copying machine is broadly divided into two kinds of systems such as a single-drum system which has one transfer drum unit and performs a plurality of times of image input and transfer operation for each color component to be transferred and a double-drum system which has a transfer drum unit for each color component and perform the transfer operation of all color components by one image input.
The single-drum system performs the scanner operation and image forming process for each color component. Therefore, since the document input operation is required to perform several times for one document, there is an aspect that the processing time for one document is prolonged. The timing for realizing the scanner operation for the next document is at the lowest the point of time when the scanner operation for the last color is finished, so that there is an aspect that there is a mechanical weak point in the document input process for increasing the speed.
On the other hand, the double-drum system performs the image forming process for an image input by one document input operation using the transfer drum unit for the ink color. Therefore, only one document input operation is sufficient for one document, so that a comparatively faster processing speed can be realized compared with the single-drum system.
However, even in the double-drum system, in one image forming, unless the image forming operations by a plurality of transfer drum units for the ink colors are all completed respectively, the next document input operation cannot be started, so that there is a structural problem imposed that particularly as compared with a monochromatic copying machine, a sufficient processing speed cannot be obtained.
Namely, unless a conventional color image forming apparatus waits until a recording medium passes through all of a plurality of transfer drum units for every color and then starts the document input operation for the next document, the data of the next document is overwritten on the previous document data, so that after a recording medium passes through all the transfer drum units for each document, the apparatus performs the document input process for the next document. However, in this case, the recording medium supply interval during copying of each document is increased and there is a problem imposed that the copying operation for unit time is not made faster sufficiently.